NAND Flash Based Content Addressable Memory

ABSTRACT

A NAND Flash based content addressable memory (CAM) is used for a key-value addressed storage drive. The device can use a standard transport protocol such as PCI-E, SAS, SATA, eMMC, SCSI, and so on. A host writes a key-value pair to the drive, where the drive writes the keys along bit lines of a CAM NAND portion of the drive and stores the value in the drive. The drive then maintains a table linking the keys to location of the value. In a read process, the host provides a key to drive, which then broadcasts down the word lines of blocks storing the keys. Based on any matching bit lines, the tables can then be used to retrieve and supply the corresponding data to the host.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Applications Nos.61/724,401, filed on Nov. 9, 2012, and 61/730,884 filed on Nov. 28,2012, and is related to the following US patent applications, all filedconcurrently with the present application and all by Steven Sprouse andYan Li: a pair of applications entitled “NAND FLASH BASED CONTENTADDRESSABLE MEMORY” and having attorney docket numbers 0084567-784US1and 0084567-784US3; and a pair of applications entitled “KEY-VALUEADDRESSED STORAGE DRIVE USING NAND FLASH BASED CONTENT ADDRESSABLEMEMORY” and having attorney docket numbers 0084567-786US1 and0084567-786US2, which are all hereby incorporated in its entirety bythis reference.

FIELD OF THE INVENTION

This invention relates generally to non-volatile memory drives and, morespecifically, to such drives using a content addressable memory orstorage (CAM or CAS) based on flash memory of a NAND type architecture.

BACKGROUND OF THE INVENTION

Content addressable memories, also known as associative memories, aredifferent from standard memories in the way that data is addressed andretrieved. In a conventional memory, an address is supplied and the datalocated at this specified address is retrieved. In contrast, in acontent addressable memory (CAM), data is written as a key-data pair. Toretrieve the data, a search key is supplied and all the keys in thememory are searched for a match. If a match is found, the correspondingdata is retrieved.

Content Addressable Memories, or CAMs, can be implemented in severalways. In one sort of embodiment, a CAM is implemented using aconventional memory and an associated CPU which searches through thememory to find a matching key. The keys in the memory may be sorted, inwhich case a binary search can be used; or they can be unsorted, inwhich case a linear search is used. A CAM can also be implemented as asemiconductor memory, where every memory location contains an n-bitcomparator. When an n-bit key is provided, each entry in the CAM willcompare the search key with the entry's key, and signal a match if thetwo are equal.

SUMMARY OF INVENTION

A content addressable memory (CAM) is based on a flash memory of a NANDtype architecture. Keys are written along bit lines. The keys can bewritten as key, inverted key pairs where, for example, the key is placedon one bit line and the inverted key on another word line or both thekey and its inverse are written on the same bit line. To locate a key, acorresponding search key pattern is broadcast to all of the bit lines ofa block by setting the word lines to a corresponding set of high and lowread voltages. In addition to a bit line based CAM, keys can also becompared along the word line direction, allowing for large key size,such as 1 KB to 16 KB. In the word line based content search, the keyswill be entered into data latches inside NAND, with an internalcomparison done on all word lines and all the blocks internally on theNAND chip.

This type of NAND Flash base CAM can be used for a key-value addressedstorage drive. The device can use a standard transport protocol such asPCT-E, SAS, SATA, eMMC, SCSI, and so on. A host writes a key-value pairto the drive, where the drive writes the keys along bit lines of a CAMNAND portion of the drive and stores the value in the drive. The drivethen maintains a table linking the keys to location of the value. In aread process, the host provides a key to drive, which then broadcastsdown the word lines of blocks storing the keys. Based on any matchingbit lines, the tables can then be used to retrieve and supply thecorresponding data to the host.

A first set of aspects concern a memory circuit. The memory circuitincludes an array of non-volatile memory cells arranged into a NAND typeof architecture, including a plurality of M NAND strings, each having aplurality of N memory cells connected in series, and a plurality of Nword lines spanning the M NAND strings. Each of the N word lines isconnected to a corresponding one of the N memory cells of the array,where M and N are integers. Word line driving circuitry is connectableto the second plurality of word lines, whereby each of N word lines canbe concurrently and individually be set to one of a plurality of datadependent read values corresponding to a data pattern. Sensing circuitryis connectable to the M NAND strings to individually determine those ofthe M NAND strings that are conducting in response the word line drivingcircuitry applying a data pattern to the N word lines.

Other aspects relate to a method of operating a memory system, where thememory system includes an array of non-volatile memory cells arrangedinto a NAND type of architecture, including a plurality NAND strings anda plurality word lines spanning the NAND strings, each of the word linesconnected to a corresponding one of the memory cells thereof. The methodincludes receiving a search data pattern from a host device to which thememory system is connected and biasing a first plurality of the wordlines according to the search data pattern. Those of the NAND stringsthat conduct in response to the first plurality of the word lines biasedaccording to the search data pattern being applied thereto aredetermined concurrently.

Further aspects concern a method of operating a non-volatile memorysystem. The method includes receiving on a controller circuit of thememory system a plurality of multi-bit data keys and, for each of thekeys, an associated set of data from a host to which the memory systemis connected. The data keys are formed into a plurality of data pages,each of one or more keys, and the data pages are written into acorresponding plurality of word lines into a first non-volatile memoryarray of the memory system. The associated sets of data are written intoa second non-volatile memory array of the memory system. The controllercircuit maintains a mapping between each of the keys and the associatedsets of data.

Yet more aspect relate to a method of operating a memory system, wherethe memory system including a first array of non-volatile memory cellsarranged into a NAND type of architecture, including a plurality NANDstrings and a plurality word lines spanning the NAND strings, each ofthe word lines connected to a corresponding one of the memory cellsthereof, wherein the first array is written as pages of data alongselected word lines. The method includes receiving a plurality of datapatterns from a host device to which the memory system is connected andstoring the received data patterns in a RAM memory of the memory system.On the memory system, subsequently arranging the data patterns stored inthe RAM memory are subsequently arranged into pages of data so that whenwritten into first array, the data patterns arranged to be orientatedalong NAND strings of the first array and the pages of data are writteninto the first array.

Additional aspects concern to a memory system including a buffer memorysection, a non-volatile memory section and control circuitry. Thenon-volatile memory section is connectable to the buffer memory sectionand includes a first array of non-volatile memory cells arranged into aNAND type of architecture, including a plurality NAND strings and aplurality word lines spanning the NAND strings, each of the word linesconnected to a corresponding one of the memory cells thereof, whereinthe first array is written as pages of data along selected word lines.The control circuitry is connected to the buffer memory section and thenon-volatile memory section, wherein the control circuitry can: storedata keys received from a host to which the memory circuit is connectedin the buffer memory; subsequently form the data keys stored in thebuffer memory into pages of data such that when written into thenon-volatile memory section, the data keys are oriented along NANDstrings of the first array; and transfer the data pages to thenon-volatile memory section to be written into the first array.

Various aspects, advantages, features and embodiments of the presentinvention are included in the following description of exemplaryexamples thereof, which description should be taken in conjunction withthe accompanying drawings. All patents, patent applications, articles,other publications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of terms between any of the incorporated publications,documents or things and the present application, those of the presentapplication shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a NAND array used as a CAMmemory.

FIG. 2 is a schematic illustration of the network of some of theelements to supply the word line in a NAND array for conventionaloperation.

FIG. 3 is a schematic illustration of the network of some of theelements to supply the word line in a NAND array for CAM operation.

FIG. 4 shows one embodiment for how keys can be written along bit linesof an NAND array and searched.

FIG. 5 given some detail on how a key/inverse pair from FIG. 4 isprogrammed into a pair of NAND strings.

FIGS. 6A-C shows another embodiment for how keys can be written alongbit lines of an NAND array and searched.

FIG. 7 shows an exemplary encoding of 2-bits per cells for four statememory cell operation.

FIG. 8 shows how the data states and the complementary data used for theinverted keys correspond in the 2-bit per cell example.

FIG. 9 shows an example of how a key would be encoded onto a 4 cell NANDstring on bit line BL and its inverse on bit line BLB.

FIG. 10 illustrates the process of matching of content in word linedirection.

FIG. 11 illustrates how the position of a conducting bit line can beused as an index in to another table that can be used to retrieve dataassociated with the target key.

FIG. 12 schematically illustrates how a key-value pair is stored in aNAND based CAM and how the value is accessed using the key.

FIG. 13 illustrates a memory arrangement for transposing the data keys.

FIG. 14 represents a first hardware embodiment for transposing datausing a FIFO-type structure.

FIG. 15 represents another hardware embodiment for transposing data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Content Addressable MemoryBased on NAND Flash Memory

The following presents a method of using a Flash based NAND memory arrayas a content addressable memory (CAM) that can be realized in bothbinary and ternary embodiments. As described in more detail below, keyscan be programmed along the bit lines of a block. The search key is theninput along the word lines of the blocks, so that a bit line on which acorresponding key has been programming will be conducting. This allowsfor all the keys of a block to be checked at the same time.

The typical way by which a NAND memory array is read is that data isread out a single word line (or portion of a word line) at a time, withthe non-selected word lines along the NAND strings being bias so thatthey are fully turned on regardless of the data state, removing thenon-selected memory from affecting the read operation. In this way, thedata content of the memory is read out a page (the unit of read) at atime. In contrast, to use a NAND flash memory as a content addressablememory, all of the word lines are set a specific data dependent value,where the data is the key, and the memory determines which bit linesthen conduct, thereby determining particular bit lines corresponding tothe input key, rather that the data of individual cells. An operationwhere sensing voltages are applied to multiple word lines in the contextof an enhanced post-write read operation is given in U.S. patentapplication Ser. No. 13/332,780 filed on Dec. 21, 2011, (and which alsopresents more detail on NAND flash memory in general); however, even inthat case only a few of the word lines receive a sensing voltage. Also,in prior art NAND memories, data was aligned along word lines, wheredata pages (for both read and write) are aligned along the word lines.Here, data is aligned along bit lines and many, or even all, of the wordlines along the bit lines can receive either a high voltage sufficientto turn on a cell in a programmed state, or a low voltage sufficient toturn on a cell in the erased state. The following discussion will usethe EEPROM based flash memory as the exemplary embodiment, but othermemory devices having a NAND type of architecture, including 3D NAND(such as described in T. Maeda et al., “Multi-stacked 1 G cell/layerPipe-shaped BiCS flash memory”, 2009 Symposium on VLSI Circuits, pages22-23) for example, can also be used.

In a binary, EEPROM based flash memory, in a write operation each cellis either left in an erased state or charge is placed on the cell'sfloating gate to put the cell in a programmed state, which here arerespectively taken as the 1 and 0 states. When a low value for the readvoltage is applied to its control gate, only a cell in the erased, or 1,state will conduct. For cells in the programmed, or 0, state, a highvalue of the read voltage needs to be applied to the control gate for acell to conduct. The keys will be arranged along bit lines of a block ofthe memory array. Since a cell in the 1 state will conduct for eitherread voltage, each key needs to be written twice, in inverted andnon-inverted form. As discussed below, this can be done by writing thetarget key along one bit line and its inverse along another, or writinghalf the bit line with the (non-inverted) target key and the other halfof the bit line with the inverted target key. More key info can becompressed into the NAND chain using multiple bits programming. Forexample, in a 2-3 bits per cell case, the key can be sorted in thecontroller RAM and the bits will be programmed as lower, (middle) orupper pages. The following discussion will mostly be given in terms of abinary embodiment, with some specifics of the multi-state case discussedfurther down.

The general concept can be illustrated by FIG. 1. Target keys Key 0, Key1, . . . are programmed down bit lines BL0, BL1, of a NAND block. Datais programmed in a separate location that can be indexed by the targetkey's column address number. To search the block for a key, the searchkey is broadcasted on the block's word lines by setting all of the wordlines according to the either high or low read voltage according to thesearch key. (In addition to setting the word line voltages according tothe key, the select gates at the end of the NAND string will also needto be turned on.) Each BL effectively compares itself to the WL keypattern for all of the bit lines in the block at the same time. If thebit line key matches the search key, the whole of the bit line will beconducting and a “1” will be read out. (Note that, as discussed furtherin the following, this is discussion is somewhat simplified for thereasons discussed in the last paragraph.) Once the column index of thekey is found, it can be used to fetch the corresponding data from a“data” block. The key can be the hash code of the data page that willlead to the right data page by the column address of the matched NANDchain. For content matching applications, such as data compression orde-duplication, each 16 KB, say, of content can generate a correspondinghash code that can be stored along the NAND chain. If the key along theNAND chain is matched, then the data page will be compared with thecomparing data along the word line to avoid hash collision cases. Inother cases, the content along the word line may not be a hash value,but characteristics of the data elements that can be searched as a keysto data; or the bits lines themselves main be the elements of the datathemselves, rather than a pointer to a data base.

Under the arrangement illustrated by FIG. 1, all of the bit lines of thearray, and consequently all of the keys, are searched at the same time.In arrays that do not use an all bit line type of architecture, thenumber of keys searched simultaneously would be the number of bit linesensed in parallel, such as half of the total in an odd-evenarrangement. The size of the key is the number of word lines. Inpractice, these maximum values of the keys will typically be somewhatless, since some column are usually set aside for defects, for instance.

As noted above, since a memory cell in either the 0 or 1 state willconduct for a high read voltage, the key will need to be entered twice,both non-inverted and inverted. This can be done by either programmingthe target key on two bit lines, reducing the number of keys by half, orprogramming both versions of the key on the same bit line, reducing thekey size by half. However, given the size of available NAND blocks, evenwith these reductions the number of keys that can be checked in parallelis quite large. Relative to some other memory technologies, NAND flashmemory has relatively large latencies in its operation, but in manyapplications this would more than be offset by the number of keys (bitlines) that can be checked in parallel (128K, for example). The processcan all be done on-chip and, as only the bit lines that meet thematching case conducting current, with relatively low power consumption,so that compared to toggling out all of the data from the memory anddoing the compare in the controller, it is a process of relatively lowpower and higher speed.

Looking at some implementation detail, an exemplary embodiment can bebased on a flash memory where the indices are saved on the 128 Gb NANDchains. An all bit line (ABL) architecture is used where one sensingoperations will perform a match operation on all of the indices on ablock at the same time. Extra column redundancy is included to avoid anybad columns (more detail on such redundancy and the accessing ofcolumns, as well as flash memory in general, can be found in thefollowing US patent publication/application numbers: US-2005-0141387-A1;US-2008-0266957-A1; US-2011-0002169-A1; US-2010-0329007-A1; Ser. No.13/463,422; and Ser. No. 13/420,961.) Two copies of the same data, Dataand Data Bar, are written into the NAND chain. In the example, thisallows for 16 KB/2/2=32000 sets of information with a 128 bit key.

When writing in the keys, these will be typically writing in on a pageby page basis, although in memories that allow it, partial pageprogramming can be used to write part of the keys, with more addedlater. Such partial page programming is typically more limited formulti-states implementations than in binary blocks. As one example, thedata can be shifted on to the memory and the inverted data can begenerated on the memory to save effort on the controller for these datamanipulations, where the data and data bar can be written withoutshifting in the data twice, the data written first, the inversegenerated is then written. Both the keys and the data can be input intothe memory system, or in some cases the keys could be generated on thememory system by the controller from the data, such as by generatinghash values from the data to use as keys. If the keys are to be sortedbefore being written along the bit lines, this will typically be done onthe controller due to the amount of data involved, such as multipleblocks' worth of data. For example, the data could initially be writtenin a particular area, say die 0, plane 0, blocks 0-15, and then sortedand written into the blocks having been sorted to the block level.Alternately, the keys could be assembled in RAM (either on thecontroller or on a separate chip) or cache NAND memory (such asdescribed in U.S. provisional application No. 61/713,038) before sortingthem to the desired level of granularity and writing them into a set ofblocks.

As discussed further below, the data/data bar pairs can be written ontwo bits lines or on a single bit line. When the data/data bar pairs arewritten on two bit lines, such as discussed with respect to FIG. 4, thepairs can be written next to each other or in other patterns, such aswriting the data bit lines in one area and the inverted data bit linesin another zone. When both parts of the pair on written on the same bitline, as discussed below with respect to FIG. 6A, they can be written ina top/bottom format or interleaved. For example, when the data andinverted data are interleaved to alternates down the word lines, thishas the advantage of that at most two elements in a row are the samedown the bit line; further, interleaving can lead to efficient datatransfer on to the memory as first a page of data is transferred on thememory and the next page can just be generated in the latches byinverting all the bits, as the next page is the inverted data of thefirst page.

The matched index can then be linked to other data corresponding to thedetermined column address; for instance, the keys could be a hash value,such as from a Secure Hash Algorithm (SHA), used to point to the actualdata that can also be stored elsewhere on the memory itself. All thematching can be done inside of the NAND chip and, when the match isfound, the column address can also be transfer out if needed or just thedata, if also stored on the NAND chip, can be transferred out.

To efficiently implement the use of a NAND array as a CAM memory, sochanges can be made to the word line driving circuitry. To broadcast asearch key down the word lines of a block, in addition to turning on theselect gates on either end of the NAND strings, each word line of theblock needs to be set to either the high or low read voltage accordingto the search key. This is in contrast to typical NAND operation, whereonly a single word line at a time is selected for a read voltage, withall of the other word lines receiving a pass voltage sufficient toremove them from influencing the sensing regardless of their data state.

FIG. 2 is a schematic illustration of the network of some of theelements to supply the word line in a NAND array for conventionaloperation. At 201 is the cell array for a plane of a NAND chip, with twoblocks explicitly marked out at 203 and 205. Each block's word lines arefeed by a word line select gate WLSW 213 or 215 as controlled fromselect circuitry at 217. The bit lines are not indicated, but would rundown to the sense amp block S/A 207. The various control gate voltageCGI are then supplied to the select gates 213 and 215 from the driversCG drivers 231 and UCG drivers 233 and 235 by way of switches 223 and225, respectively. In the exemplary embodiment shown here, a block istaken to have 132 word lines, where a pair of dummy word lines areincluded on both the drain and source sides of the NAND strings. The UCGDrivers 233 and 235 are for supplying the pass voltages used onunselected word lines during program, (standard, non-CAM) read or verifyoperations. As this level is used on the large majority of word lines,these can be lumped together for a single driver. The selected controlgates are biased to VPGM at program, CGR voltage at read or verify. InFIG. 2, CGI<126:1> is the decoded global CG lines. CGI<0> and CGI<127>,that are here biased differently from other 126 word lines due to edgeword line effects. The dummy word line bias CGD0/1 is for the drain sidedummy word lines and CGDS0/1 is for the source side ones.

For a typical NAND memory operation, only a few word lines at a time areindividually biased. In addition to a selected word line, adjacent oredge word lines may receive special bias levels to improve operations.Consequently, existing word line drivers are arranged so that they canonly take care of a handful of word lines. With logic changes, it may bepossible to drive up to perhaps two dozen or so word lines. However, todrive all the word lines of a block (here 128, ignoring dummies) willrequire additional analog drivers. FIG. 3 illustrates some of thesechanges.

The array 301, blocks 303 and 305, select circuitry 317, CG Drivers 331,and switches 313 and 315 can be the same as in FIG. 2. The additionalword line drivers are shown at 343 and 345 and can supply the word linesthrough respective switches at 353 and 355. In each of 343 and 345, thelevel shifter HVLSHIFT receives the voltage VREAD and a digital valueDFF(0/1) for each word line. The level shifter then converts the digitalvalues of 0, 1 for the broadcast key to the analog high and low wordline levels. As the memory cells will still need to be written (bothprogrammed and program verified), the other circuit sketched out in FIG.2 will still be present, though not shown in FIG. 3 to simplify thediscussion. It may also be preferable to make some changes to thesensing circuitry S/A 307 to more efficiently perform the XOR operationdescribed below between the pairs of bit lines holding a key and itsinverse.

FIG. 4 shows the encoding of the keys along bit lines, where the key isentered twice, in non-inverted and inverted form. Here the bit lines arelabeled BL for the non-inverted key and BLB for the inverted version.Here the pairs are shown as being adjacent, although this need not bethe case, but will typically make XOR-ing and keeping track of dataeasier. Also, this arrangement readily lends itself to NAND arrays usingan odd/even BL arrangement. As shown in the half of FIG. 4, forreference a key of all 1s is written along BL1 and a key of all 0s iswritten along BLn, with the corresponding inverted keys at BLB1 andBLBn. For the defective bit lines, the bit line either stuck “0” orstuck “1” regardless of the word line voltage bias. The XOR resultsbetween the two read results will always yield “1”. The BL and BLB datapattern will eliminate the defected bit lines from yielding matchresults mistakenly. In this example, only seven word lines are used. Amore interesting key of (1001101) is entered on BLn+1, with its invertedversion at BLBn+1, as also illustrated in FIG. 5.

FIG. 5 shows the two corresponding NAND strings, where 0 is a programmedcell, 1 a cell left in its erased state, the cells being connected inseries down the NAND strings to the common source line CELSRC. To searchfor this key, it is encoded as low read voltage for the 0 entries andhigh read voltage for the 1s. The search key is shown at the left of thetop of FIG. 5. When put onto the word lines, this correspondingly findsthat BLn+1 is conducting (and BLBn+1 is non-conducting), as shown by the“c” (and “nc”) in the sense 1 row. However, BL1 and BLBn are also bothconducting, as a cell in the 1 state will conduct for either read value.

The second sensing (these can be performed in either order) is then madewith the search reversed. Although BL1 and BLBn are still conducting,the result from the key actually sought has changed: BLn+1 is nownon-conducting and BLBn+1 conducts. By taking the result of the tworeads and XOR-ing them, the sought key will give a 0 on thecorresponding bit line and also on its inverted version. Consequently,by searching for the 00 pattern in the XOR data, the output columnaddress can be found and the corresponding data block accessed. Underthe sort of embodiment used in FIG. 4, two reads are needed for thepattern match and internal pattern detection on the NAND device canjudge if there is a match. The redundancy of the BL/BLB pairs providesredundancy to help protect from bad bit lines, but a second pair canalso be kept for further protection. A copy of the key can also be keptwith any associated data and used to check the match, where this copycan be ECC protected. Additional protection can also be provided by eachbit line including several (8, for example) parity bits, for errordetection and correction purposes, where the redundancy bit arepreferable along the same bit lines for all of the keys so that theseparity bits can either be read or taken out to the comparisons by use ofa “don't care” value applied to these word lines, as described below.For example, the data can be read when checking when checking the data,as either part of a post-write read or other data integrity check, butignored during CAM-type operations.

Generally, for both this and other embodiments described here, apost-write read can be used to insure that the keys have beensuccessfully written into the NAND memory, as any error bits couldprevent a NAND string from conducting and would give rise to “falsenegatives” when matching. If an error is found, the bad data can berewritten. In the exemplary NAND flash example, the incorrectly writtendata can rewritten to another data block and any key-datacorrespondences updated accordingly. More detail on post-write readoperations can be found in U.S. patent application Ser. No. 13/332,780and references cited therein.

In terms of performance, in the case of a 16 KB page of 128 bit keys, iftwo copies of the both the data and its inverse are stored, thecorresponds to 4 KB of keys, or 32000 keys. (As all of the word linesare sensed at once, so that here, a “page” involves a sensing of all theword lines of a block rather than a single word line.) If this page of32000 keys is sensed in 50 us, this is a rate of 0.64 GC (Giga-compares)per second per plane. If four planes are sensed in parallel, this canlead to 2.56 GC/s at a consumption of about 200 mW.

FIG. 6A illustrates a second embodiment for how the key can be storedalong a bit line. In this case, both the key and its inverse are writtenonto the same bit line. For a given block, this means that the maximumkey size is only half the number of word lines, but this allows for thesearch key and inverted key to be broadcast at the same time.Consequently, the search can be done in a single read.

Referring to FIG. 6A, this shows 14 different word lines with the keysentered in the top half and the inverted versions of these same keysentered in inverted form in the bottom half of the same bit line. Thus,taking the bit line at D7, rows 1-7 hold a 7 bit key, and rows 8-14 theinverted version of the same key. (Although arranged similarly to FIG.4, in FIG. 6A the top and bottom halves represent 14 different wordlines where the top-bottom division is the key/inverted key boundary,whereas in FIG. 4, the top and bottom are the same seven word linesrepeated twice for two different sensing operations.) For comparisonpurposes, the keys shown in FIG. 6A are the same as in FIG. 4, with thebit line of D7 holding the sought for key in the top half and itsinverse in the bottom half, and D8 holding the inverted key so thatthese two halves are switched.

To search for a key, the search pattern is then broadcast on the tophalf word lines and its inverse on the bottom half word lines. Any bitlines with a matching keys, in this case D7, will then conduct, as shownat bottom where “nc” is non-conducting and “c” conducting. If redundancyis desired, the non-inverted version can also be programmed in as at D8and then detected by broadcasting the non-inverted search key, and thebit lines reads searched for a 11 pattern, which can then be output as adata pointer. If further redundancy is wanted, the key or key/inversepair can be written into the array a second time and parity bits canalso be included, much the same way as discussed for the embodimentsbased on FIG. 4. The defective bit line should be isolated withisolation latch and not used. If some defect shows up as a stuck “0”, itcan potentially generate the “false” match. In this case, the datacontent should be compared in order to confirm whether this is a realmatch or a false match. The other most common reliability issue is thatsome cells may have lost some charges after some time, that will alsoproduce a “false” match. Then a content match check will eliminate the“false” match error. The word line voltage bias can be budgeted a littlehigher to avoid “missing” a match, which is very harmful error. A“false” match can be double checked with the content check.

FIG. 6B schematically illustrates the key/inverse pairs along NANDstrings. Two strings are shown (for bit lines BLn and BLm) each having adrain and source select gate (SGD, SGS) on either end, where the sourceends are then connected along the source line CELSRC. In between are thememory cells on the strings connected in series. In this example, thestings has cell capacity to hold a 48 bit key, its 48 bit inverse, andsome parity bits. Although shown here with the key along the first 48word lines followed by the inverse along the next 48 word lines, moregenerally they can interleaved in various ways; for example, each of thekey bits can be followed it inverse in the next word line as, whenprogramming, this allows for a page to loading in and written, afterwhich the programming data can be inverted in the latches and writteninto the next word line. The parity bits can also be variously locatedalong the NAND string, although having them grouped can lead to easierdecoding when searching the keys.

Each of bit lines BLn and BLm show a portion of a key along fouradjacent word lines and the corresponding four adjacent word linesholding the inverse. To search the keys of the block, the word lines arethen biased according to the search key, where the high sensing voltageused to checking for “0” values and the low sensing voltage to check for‘1” values. The high value is here taken as VREAD, and can be the sameused in a typical NAND memory for non-selected word lines, and the lowsensing values is labeled as V0. The select gates will also need to beon and VREAD should also be applied to the word lines holding paritybits as these as used for data integrity checks and are not meant factorinto key search operations.

To make the stored keys more robust, the memory can shift the sensingmargins to favor “false” matches rather than misses. (Similarly, theprogramming parameters can be shifter relative to those typically used.)The “false” matches can be examined by the data check later to helpremove any false positives. A duplicated key can be used to check forpreventing error, where these duplicates can be stored on other NANDstrings, with the associated data, or other locations on the system.Relative to a standard NAND memory, this arrangement will need to addextra circuitry, as described with respect to FIGS. 2 and 3.

Rather than sense the search for the full key (or key/inverse) in asingle sensing, a partial key can be searched, allowing the fullkey/inverse matching to be done incrementally. This can allows for theless independently settable word line levels, resulting in lesscircuitry changes relative to a standard NAND memory, but it can requiresome logic changes. The full key/inverse can be searched sequentially,where each subsequent sensing will be judged based on previous sensingresults. For the example of FIG. 6B, rather than check all 24+24 wordlines of the key/inverse in one go, a partially key check of, say 24bits at a time can be done: if no matches are found, the process canmove on to any other blocks holding keys; if a match is found, a secondpartial key can be checked, and so on. The subsequent checks can eitherdo all of the NAND string again and compare the results of the partialsearches, or only check those which have conducted in the previouspartial key matches. FIG. 6C illustrated such a partial key comparison,where only 24 bits of the 48 bits in the key are being checked. Theother bits of the key and its inverse are then set to the “don't care”value, as shown at the corresponding bits of the inverse that are set atVREAD.

As each key is written in twice (non-inverted, inverted) on a bit line,a block with 128 word lines can hold 64 bit keys, while 128 bit keyswould need blocks of 256 word lines. Also, it should be noted thatalthough the key/inverted keys are here shown as being writtenrespectively into the top half/bottom half of the word lines. Moregenerally, the keys and inverse pairs could be interleaved in anydesired fashion, as long as it was consistent for all of the keys in theblock; however, this would require keeping track of the arrangement. Theinterleaved pattern along the NAND chain may be preferred since the datacan be inversely program in another WL without loading the data again.There are some other coupling effect may also benefit from interleavingthe inverted and non-inverted data on adjacent word lines. In terms ofperformance for this type of embodiment, for a 16 KB page of 64 bitkeys, if a duplicate key/inverted key pair is kept, this is 8 KB, or64,000 keys. At 35 us per sensing, this gives 1.82 C/s/plane. If 4planes are operated in parallel, this is 7.3 CG/s at around 200 mW.

For either of the embodiments of FIG. 4 or FIG. 6A, the method uses theinherent “AND” functionality available in a NAND Flash memory to comparethousands of keys in a single sensing operation. This method has severalmajor advantages over traditional CPU- or semiconductor-based CAMmemories. For one, as the comparison is done “on die”, there is no needto transfer the data out of the memory. This saves both time and IOpower. Furthermore the actual comparison operations use less power thanconventional memories. As all of the bit lines are sensed at the sametime, with only the matching NAND chain is conducting current, the NANDbased CAM is highly parallel; for example, in a NAND flash memory with4×8 KB planes, (32K×8 bits/byte)/2=128K keys can be checked in one senseper die. If a sense can be done in 35 us, an even/odd sense as describedabove with respect to FIG. 4 will take 50 us. This is 128K keys in 50us, so that an entire 8 GB die (2000 blocks) could be sensed in ˜100 ms.The corresponding energy consumption is on the order of 200 mW. Toincrease performance, multiple die can be operated in parallel.

As noted in the Background section, keys can be stored in a CAM aseither sorted, in which case a binary search can be used; or asunsorted, in which case a linear search is used. This is also true of aNAND based CAM, except that as NAND based CAM can be searched at theblock level, in a sorted CAM the keys need only be sorted to thegranularity of the block or the number of blocks that are sensed inparallel. The CAM allows for a binary search, but at the block level dueto this parallelism. Even for linear searches, this degree ofparallelism can make linear searching comparable or even faster thanbinary searches for fairly large data sets. Again, for any of thesearrangements, performance here can also be improved by running multipledie in parallel.

The keys can be sorted based on a given number of most (or least)significant bits. A sorting based on significant bits is generally mostuseful when the key or content being searched is not a hash value, but aset of characteristics or data itself. In this case, the sorted data ineach block would all share a certain number of most significant bits fortheir keys.

Content addressable memory exist in both binary form, where the searchkey consists of 0s and 1s as described above, and ternary form, wherethe search key can also include “don't care” value. As discussed above,when a high read value is broadcast along a word line, all of the cellsalong that word line will conduct regardless of its state. This propertyallows for a “don't care” value to be implemented by setting thecorresponding word line to the high read voltage for both the key andits inverse; that is, when sensing with the key and its inverse (ineither the second read of FIG. 4, or the lower half of the word lines),the don't care values are set to the high read value for both the keyand its inverse, while the other values of the key are inverted asbefore.

These properties of a NAND based CAM also make it particularly suited toa number of other uses. For instance, as large numbers of keys can besearched in parallel, this allows for all copies of the same key in thesearched blocks to be determined in the process, improving efficiency ofde-duplication operations of the sort that are valuable in cleaning updata bases. The NAND structure also makes for a CAM useful as a Bloomfilter as an intersection of multiple search keys can be formed bysetting any values that differ between the keys to the high read voltagein the combined search key, which can then be used to search thehorizontally stored keys of one or more blocks in parallel.

The use of “don't care” values can also be used to perform a type of“iterative” search. This can be used the keys may have, or possiblyhave, some number of bit errors. One or a series of reduced keys, wheresome subset of values are set to “don't care” for different entries, areused for searching. Several different such keys, where a differentsubset of values is masked out in each, are then iteratively used tocheck for matches among the written keys.

Another example of where a series of reduced search keys can be employedis where the content is itself a data set, as opposed to say a hashvalue. For example, it could be values from image processing. In such acase, the content of the block could be searched to a desired number ofsignificant bits, by setting bits of lower significance to “don't care”.Similar arrangement could also be used for patterning matching of thecontent or for cases where the keys are properties of main data content.

Consequently, due to its parallelism, relatively low power consumption,or both, NAND based CAM can be used in many applications, such as database searching, voice recognition, DNA matching/genome searches,cryptography and so on. It can lend itself to CAM based indexing and canbe incorporated, for example into CAM indexed SSD systems.

So far the discussion has mainly considered the case of binary NANDmemory for CAM use. More generally, multi-state (MLC) memory can also beused; for example, in a mixed binary-MLC memory, the keys could bestored in binary memory for CAM use, while data to which the keyspointed could be stored in MLC areas. It is also possible to use MLCNAND memory for CAM, using 2 to 3 bits per cell, for example, in keymatching. Using 2 to 3 bits per cell, the NAND chain can store longerkeys. In the sort of embodiment described with respect to FIG. 6A, wherea key and its inverse are written into the same word line, a 128 cellNAND chain in binary operation can store 64 bit keys, while a 128 NANDchain with 2-bits per cell can store 128 bits keys. Similarly, 3-bitsper cell operation can store 192 bit keys.

FIG. 7 shows an exemplary encoding of 2-bits per cells for four statememory cell operation. As shown, the erased state is encoded as 11, thefirst state up (or “a” state) is 10, followed by 00 (for the “b” state)and 01 (or “c” state). The various sensing levels are also shown. FIG. 8shows how the data states and the complementary data used for theinverted keys correspond. FIG. 9 shows an example of how a key({00111001}) would be encoded onto a 4 cell NAND string on bit line BLand its complement on bit line BLB. In the MLC CAM embodiment, thesystem can use one or two word lines along the NAND chains to store theparity bits of each NAND chain in order to check on the integrity of theNAND chain. As with the binary case, manufacture defective columns canbe isolated out and more redundancy along the word lines (duplicateddata) can further protect the keys' integrity. Also, the complimentarydata shifted as illustrated in the figures to provide more sensingmargins.

In the above, keys were written down the bit lines of the array, withthe search key broadcast along the word lines, allowing the keys along ablock's bit lines to be searched in parallel. The arrangement can alsobe reversed, where NAND array can also be operated so that the contentor key matching is in the word line direction. Under this arrangement,one or more keys would be written along each word line (that can be verylong keys), an arrangement that can be useful in several differentcircumstances. Multiple short keys can be stored along the word linedirection as well. If the keys are encoded so as to have significance asa 2D array of values, this would allow for content searching in both ofthe bit line and word line directions, although the more typicalsituation would just be for content matching in the word line direction.For instance, as word lines are much long than bit lines, a word linebased CAM allows for the use of longer keys. Also, as data is written inpage along word lines, it may be more convenient, at least initially, towrite incoming key data along word lines. This would then allow for keyto be searched as written along the word lines. If desired, the keyscould then be rewritten along bit lines, where they could then besearched as described above. Due to the amount of data involved, to movethe keys from a word line orientation onto a bit line orientation, oncea block worth of the keys are assembled they could be read off of theNAND memory into the controller and then rearranged and written alongbit lines; alternately, this rotation of the keys could be performed onthe NAND device, although this will typically require the memory chip'sstructure to modified to accomplish this. This process of initiallywriting the keys along word lines, transferring the keys to thecontroller, and rearranging them to be rewritten onto bit lines can alsoinclude the sort of sorting process described above prior to rewritingthem back in a bit line orientation.

The process of matching of content in word line direction is illustratedwith respect to FIG. 10. As the system receives keys, these can beformed into pages of one or more keys and written into the memory array901 along word lines. To match content, the system inputs the matchingcontent of one or more search keys into a matching buffer or register905, which can then be used to look for duplication content along theword line. The data along a word line is read from memory array 901 intoa buffer or register 903. The memory can then perform internal matchoperations between the read data in buffer 903 and search data in buffer905, where some number of bits ignored, if desired. The ignored bits caneither be to “don't care” values, because some read error can occur onthe read. These ignored bits can be corrected with error correction inthe controller when a match is found. Typically, the smallest length ofkey/content along the word line that can be compared is 1 KB, while thelongest length of key/content that can be compared in one plane is 16KB. If the key length is smaller than 1 KB, the key can be duplicated inchunks patterns to do the pattern matching with more parallelism. Thenthe matched case will produce a group of “1” and the un-matched casewill produce 50% “1”s. Circuitry can detect if a word is all “1”s tojudge the match or miss. If there are some “0”s in a word, this word canbe discarded as a miss. In order to prevent that some read errorsproduce a miss when it should be a match, a majority voting circuitrycan be employed to choose the word with majority “1”s for matching. Somewords can be masked out by marking the isolation latch to be “ignored”.To simplify operations, it is typically preferable to write thebeginning of a file to aligned with certain columns. After finishing acompare on one word line, the next word line content can be compared ina similar sequence.

Key-Value Addressed Storage Drive Using NAND Flash Based CAM

This section considers the incorporation of the sort of NAND Flashcontent addressable memory (CAM) or content addressable storage (CAS)described above in a storage drive. Conventional storage drives, such assolid state dives or hard-disk drives (SSD or HDD), are addressed by alogical block address (LBA) to read and write data stored on theirphysical media. These employ logical to physical address translationtables to locate the data, where the address translation table is storedon flash, in DRAM, or on magnetic media and is updated on the basis ofsectors, bytes, or pages. Typical sizes for such addresses are 32, 48,or 64-bits. In some applications, such as in databases, it is desirableto have large keys (of hundreds or thousands of bits) that can address aspace much larger than the number of elements in the database. In thesecases a content addressable memory utilizing key-value pairs is used toindex the elements stored in the device.

In a content addressable memory, data is written as a key-data pair. Toretrieve the data, a search key is supplied; all the keys in the memoryare searched for a match. If a match is found, the corresponding data isretrieved. This section presents a storage drive using a Flash basedNAND array as described in the preceding section as a contentaddressable memory that is addressed using key-value pairs instead of alogical block address. This drive can provide both Binary and Ternarysearch capability, meaning that bit patterns in the key can have thevalues 1 or 0 as well as “don't care” entries. This type of NAND basedCAS drive can then be used to replace other implementations of CAM orCAS functionality, such as those employing a database, that wouldusually include a host CPU, DRAM, and storage media.

Consequently, this section applies the of operation of a NAND flashmemory as a pattern matching engine from the last section to a storagedevice that is indexed using key-value pairs instead of conventionallogical block addresses. The device can use a standard transportprotocol such as PCI-E, SAS, SATA, eMMC, SCSI, and so on. When used in apattern matching mode, the NAND cells not only store values, but canalso be used to compare their stored values with an input value. In theexamples of this section, target patterns are stored along bit lines,although the word line based storage discussed above can also be used.In the bit line example, the pattern to be matched is broadcast downword lines. If all the elements in the NAND chain match their targetpattern, the NAND chain (bit line) will conduct. The position of theconducting bit line can be used as an index in to another table that canbe used to retrieve data that is associated with the target key. This isshown in FIG. 11, that expands upon FIG. 1.

In FIG. 11, the bit lines BL0, BL1, . . . run down the columns of thearray and are written with corresponding keys Key 0, Key 1, . . . aspreviously described. The word lines are then biased according to thesearch key (here Key 2) so that it is broad to all of the bit linesspanned by the word lines. When a matching key or keys are found, thecolumn address of the bit line is then input as an index to find thedata set, also stored on the drive. A number of arrangements arepossible, where, for example, the keys could be stored in binary or MLCarrays optimized for CAM use, while the data is stored in more standardMLC arrays.

A drive using such a mechanism can then be used to search for key-valuepairs in a large search space, perform general pattern matching (usingbloom filters), or be used for determining set membership. Some of theadvantages of a drive using such a scheme include low power usage andhigh bandwidth. As data does not need to be moved from the NAND array toa separate computational module for comparison, power consumed on IOoperations is reduced. Furthermore, since only bit lines that match agiven search pattern will conduct, the NAND comparison operation is alsolow power. With respect to bandwidth, a single NAND die is capable ofdoing, say, 256K 64-bit comparisons in under 50 us, working out to under200 ps per comparison. Additionally, multiple die can be operated inparallel to increase bandwidth or to increase the effective key-length.Potentially 8 Gb (˜8 G keys) of 64-bit keys can be searched in ˜100 m sin a single die based on current design.

The idea of using a storage drive with a common interface, such as SAS,PCI-E, SATA, and so on, but which is written using key-value pairsinstead of a conventional logical addressing is consider further withrespect to FIG. 12. In the write method, the host will write Key-Valuepair (K, V) to the drive. The drive will store the Value V in a datastore at some address in the Data table of FIG. 12, as illustrate at(1). The drive will store the key value K on a bit line “i” in a blockof an array of the drive, as shown at (2) of FIG. 12. The drive willmake an entry in the block table at address i, with a pointer to thevalue V, as shown at (3). The column address which has the matching keycan be output from NAND memory from status bits.

In NAND flash memory, data is written in word line based pages. Becauseof this, as previously discussed, the keys may be initially writtenalong word lines, then rearranged to be written along bit lines, orfirst stored in RAM and then sorted into bit lined oriented keys. (Itcould also be possible for the host to have already taken care of thisbit line based orientation for the keys, although it will generally bepreferable for this operation to be transparent as seen from outside ofthe drive, with a host just providing basic key-value pairs and nothaving to engage in such data manipulations.) The controller will takecare of assigning the keys and values to physical addresses and ofdetermining the needed addressing structures to translate the key intothe corresponding data location. The key to value mapping tables can bemaintained in much the same way as the usual logical to physical mappingtables as far storing them and updating them, such as mappings usinglook up tables or based a correspondence formula. For example, thecolumn address can be mapped to metadata in the primary storage flashmanagement layers.

In a variation, if the drive itself has a key generating ability, suchas a hashing algorithm using by the controller, just the data set itselfcould be sent to the drive and the corresponding keys generated on thedrive. This would also allow a different host without the key generatingability to send data to the drive, where the key-value then be generatedand stored. Under this sort of arrangement, for a host to access a valueusing a key, the host would need to use the same key generatingalgorithm (such as from a Secure Hash Algorithm (SHA), for example) asbeing used by the drive.

Looking now at the read process, the host will send the drive a key (K)that is then used to search key blocks. As discussed in the previoussection, the key blocks may be sorted, in which case a binary search canbe used; or they can be unsorted, in which case a linear search is used.For each key block, the drive will apply the key K to the word lines. Ifa matching key exists along a bit line in the block, NAND flash willregister a “1” at the bit position “j” associated with the matching key.The value “j” can then be used as an index to the associated blocktable, as represented at (4) in FIG. 12, to retrieve a pointer, (3), tothe associated value V in the Data Table. If all key blocks are searchedwithout finding a match, the drive can return an “element not foundstatus” or error.

The CAM NAND can be incorporate into the same memory system as that inwhich the associated data is stored, such as an SSD, in which case thedata corresponding to the search key can be provided directly to thehost. In other embodiments, the CAM NAND could be a separate device usedto provide the sort of CAM-based operations described here, while theassociated data could be stored separately, in which case as address orother pointer to the corresponding data on the separated device would beprovided.

Consequently, storage drive can benefit by use of the inherent “AND”functionality available in a NAND Flash memory to compare thousands ofkeys in a single sensing operation. A storage drive of this type hasseveral major advantages over traditional CPU- or semiconductor-basedCAM memories. First, because the key comparison is done “on die”, thereis no need to transfer the data out of the memory. This saves both timeand JO Power. Furthermore the actual comparison operations use lesspower than conventional memories.

In addition to the low power and high bandwidth advantages discussed inmore detail earlier, this scheme has the advantage that write times canbe shorter if data is searched in a linear mode. Most databases spendtime and energy sorting and maintaining tables to enable fast, binarytype, search capability for when data is read. In one embodiment of thisinvention, the writes of data and keys can be done in a random fashionmaking writes times of O(1) complexity. Searches will use a linearsearch mechanism which is highly parallelized but is still O(N)complexity. This is less efficient than the O(LogN) of most binarysearches and is a tradeoff between insertion time vs. lookup time. Also,it should be noted that even when data is stored in a sorted manner, thehigh degree of parallelism in searching mean that the sorting only needsto be done to the level at which the search is done, namely to thegranularity of block or number of blocks searchable in parallel.

The sort of NAND flash base CAS drives can be applied to a number ofapplications, including those described in the previous section. One setof examples of these exemplary applications is for de-duplication usingpattern matching (CAM) NAND to store the hash keys. Incoming data can besent through the hash function to generate the content relatedfingerprints. The fingerprints can then be searched with the existinghash keys to see whether the data already exists in the data storage. Ifit does already exist, no write action is taken; but if the data doesnot yet exit, then the new data will be written into the storage. Thede-duplication can be done when the data is backing up, during garbagecollection operations of the primary storage, or in-line as the datacomes in from host. Another application is for virtual memorymanagement, which can be done similarly to de-duplication. The drive canalso be applied to the Human Genome, where the drives stores signaturesin the CAM NAND so that any segment of the DNA sequence can be searched.The drive also lends itself to parallel computing where, a mathematicalNAND function can be done inside of the NAND memory.

As noted above, the CAM NAND operation has the keys oriented along bitline, whereas NAND memory written along word lines. Consequently, as thekeys come in from a host, they need to be accumulated in a buffer memoryof some sort, transposed to a bit line orientation, formed into pages(including adding any inverse keys as needed), and transferred to theNAND device for writing. This is illustrated schematically in FIG. 13.

As shown in FIG. 13, a host 1301 (a PC, for example) can take the datafiles and generate the corresponding keys, such as using a Secure HashAlgorithm (SHA) to generate a 64 bit hash key, which can then betransferred over to a buffer memory 1303 on the memory system, where thekeys can be accumulated. The transposing buffer memory 1303 is used toalign the keys for writing in the NAND CAM memory 1305. Once asufficient number of keys, say 4 MB keys for a NAND memory 1305 with 4MB blocks, the data can be transferred over for programming as pagesalong the word lines. FIGS. 14 and 15 give some examples of hardwareimplementations for the transposing buffer memory, but this can beimplemented in various other ways, such as by use of a fieldprogrammable gate array (FPGA). For example, a blocks worth of keyscould be accumulated in an FPGA and then read out a word line at a timeand transferred over to the CAM NAND for writing.

FIG. 14 is a schematic illustration of a hardware implementation of thetransposing memory in FIFO style. The data can come in as, say, 64 bitskeys or indices and is saved in column-oriented 64 bits registers. Theregisters are chained into a FIFO arrangement so that when a new keycomes in, the previous keys shift over by one column to the right. Oncethere are enough keys for one page (i.e., 8 KB to 16 KB for a typicaldesign) to be programmed into the CAM NAND memory (or early, if smallernumber of keys are to be used), the pages are shifted over to the NANDfor programming into the array there. Under this arrangement, ifdesired, the keys may be searched while still in the FIFO before beingprogrammed, as the keys can be shifted out one at a time for comparison.

FIG. 15 is a schematic illustration of another hardware implementationfor transposing the data keys using more of a RAM style arrangement. Thedata can come in as, for example, 64 bit keys or indices and be saved in64 bits registers, being accumulated in a relatively small, 16×64 array1509 in latches. The 16×64 bits of the small array 1509 can then beshifted over a bus 1507 a word (16 bits) at a time into the RAM 1503.After shifting the 64 words into RAM, the small array 1509 canaccumulate next 16×64 bits. This process can continue until the RAM 1503is full or it is otherwise desired to write in the keys, at which pointis programmed in the CAM NAND memory. Under the arrangement of FIG. 15,if it is desired to search the keys before they are written into the CAMNAND, another RAM buffer storing the keys without transpose can be keptfor this search purpose.

CONCLUSION

The foregoing detailed description has been presented for purposes ofillustration and description. It is not intended to be exhaustive orlimiting to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. The describedembodiments were chosen in order to best explain the principles of thedisclosed technology and its practical application, to thereby enableothers skilled in the art to best utilize the technology in variousembodiments and with various modifications as are suited to theparticular use contemplated.

1. A method of operating a memory system, the memory system including anarray of non-volatile memory cells arranged into a NAND type ofarchitecture, including a plurality NAND strings and a plurality wordlines spanning the NAND strings, each of the word lines connected to acorresponding one of the memory cells thereof, the method comprising:receiving a search data pattern from a host device to which the memorysystem is connected; biasing a first plurality of the word linesaccording to the search data pattern; and concurrently determining thoseof the NAND strings that conduct in response to the first plurality ofthe word lines biased according to the search data pattern being appliedthereto.
 2. The method of claim 1, wherein the search pattern caninclude one or more “don't care” values.
 3. The method of claim 1,further comprising providing the result of the determining to the host.4. The method of claim 1, wherein the memory system maintain acorrespondence between the NAND strings and sets of data stored on thememory system, the method further comprising: providing to the host thesets of data corresponding to those of the NAND strings that conduct inresponse to the word lines biased according the search data patternbeing applied thereto.
 5. The method of claim 4, wherein the memorysystem maintains the correspondence between the NAND strings and sets ofdata stored on the memory system in a non-volatile memory section of thememory system other than said array.
 6. The method of claim 4, whereinthe memory system maintains the correspondence between the NAND stringsand sets of data stored on the memory system as metadata associated withthe location of the NAND strings in said array.
 7. The method of claim6, wherein the correspondence is based upon equation relating thelocation of the NAND strings and the associated data sets' location. 8.The method of claim 1, where the first set of word lines is all of theword lines spanning the NAND strings.
 9. The method of claim 1, wherethe first set of word lines a subset of all of the word lines spanningthe NAND strings, the first subset of the word lines being biasedaccording to a first portion of the search data pattern, the methodfurther comprising: in response to determining that one or more of theNAND strings conduct in response to the first plurality of the wordlines biased according the search data pattern being applied thereto,biasing a second plurality of word lines biased according to a secondportion of the search data pattern; and concurrently determining thoseof the NAND strings that conduct in response to the second plurality ofthe word lines biased according the second portion of the search datapattern being applied thereto.
 10. The method of claim 1, where thefirst set of word lines a subset of all of the word lines spanning theNAND strings, the first subset of the word lines being biased accordingto a first portion of the search data pattern, the method furthercomprising: in response to determining that none of the NAND stringsconduct in response to the first plurality of the word lines biasedaccording the search data pattern being applied thereto, performing apattern search operation on a different set of NAND strings using thesearch data pattern.
 11. The method of claim 1, wherein biasing a firstplurality of the word lines according to the search data patternincludes: biasing a first subset of the word lines as specified by thedata search pattern; and biasing a second, distinct subset of the wordlines to an inverted version of the values used for the first subset.12. The method of claim 11, wherein the first and second subsets of wordlines are interleaved.
 13. The method of claim 11, wherein the datasearch pattern includes one or more “don't care” that are biased to thesame level in both the first and second subsets.
 14. The method of claim1, further comprising: biasing the first plurality of the word linesaccording to the inverse of the search data pattern; concurrentlydetermining those of the NAND strings that conduct in response to thefirst plurality of the word lines biased according to the inverse of thesearch data pattern being applied thereto; and comparing those of theNAND strings that conduct in response to the first plurality of the wordlines biased according to the search data pattern being applied theretowith those of the NAND strings that conduct in response to the firstplurality of the word lines biased according to the inverse of thesearch data pattern being applied thereto.
 15. The method of claim 14,wherein the data search pattern includes one or more “don't care” thatare biased to the same level in both the search data pattern and theinverted version of the search data pattern.
 16. The method of claim 1,wherein the memory cells of the array along one or more of the wordlines store parity data, and wherein, when biasing the first pluralityof the word lines, word lines holding parity data are set to beconducting.
 17. The method of claim 1, wherein, for the NAND strings,the memory system maintains a copy of each of the data patterns storedthereon in non-volatile memory and for each of the maintains acorrespondence between the copy of the data pattern and the location ofthe NAND string.
 18. The method of claim 17, wherein the copies of datapatterns are stored with metadata associated with the data patterns. 19.The method of claim 17, wherein each of the data patterns is stored withcorresponding error correction code.
 20. The method of claim 17, furthercomprising: for those of the NAND strings determined to conduct,comparing the corresponding data pattern as stored on the NAND stringwith the copy thereof.
 21. The method of claim 1, wherein the array ofnon-volatile memory cells is of a 3D-type of architecture.